In conventional radio frequency communications systems, communication between two parties typically occurs over a preselected channel corresponding to a single operating frequency. For example, a conventional police radio communication system may include multiple mobile transceivers and a stationary central unit at a control site. Each mobile transceiver includes circuitry for transmitting and receiving over a single channel corresponding to a single operating frequency. Alternatively, each mobile transceiver may include a channel selector switch for selecting one of several channels, i.e., frequencies, for communication. However, each communication conventionally occurs over a single frequency.
In a typical communication, a police officer may select a channel on a mobile transceiver (if channel selection is provided), for example 800 MHz, and transmit voice signals at 800 MHz to a dispatcher at the control site. The dispatcher receives the communication and responds by transmitting at 800 MHz or at another fixed frequency to which the mobile transceiver is tuned for receiving. Since the mobile transceiver is tuned to the control site transmission frequency, the police officer hears the response from the dispatcher. The police officer may then reply to the communication from the dispatcher on the selected transmitting frequency. In this manner, communication occurs over a single channel corresponding to a single operating frequency. Alternatively, transmissions from the mobile transceiver and the control site transceiver may occur on two respective frequencies in a duplex operation. Listening to these single frequency or dual frequency transmissions is easily accomplished with a conventional fixed frequency receiver.
In past years, frequency scanning receivers have permitted monitoring of multiple frequency transmissions, particularly on public safety and public service channels. The typical scanning receiver has multiple tuners or an electronically driven tuner so that each locally used frequency can be received, one transmission at a time. The receiver automatically steps from frequency to frequency, i.e., scans, stopping or dwelling on a channel in use until the transmissions on that channel cease. Then, the frequency scanning resumes. Alternatively, the listener may skip or modify scanning to listen to a particular channel even if there are interruptions in transmission on that channel.
In trunked radio communications systems, communications between two parties may occur on multiple channels, i.e., on differing frequencies, for respective transmissions. For example, in a trunked system, a police officer may transmit voice information from a mobile transceiver to the dispatcher at the control site at a first frequency. The dispatcher may respond on a second frequency. The police officer may reply on a third frequency, and so on. Digital signaling between the mobile transceiver and the control site on a separate control channel supplies each unit with the next transmission frequency for tuning both the transmitter and receiver sections of the transceivers.
Conventional scanning radio receivers are unable to monitor communication in a trunked system coherently. Scanning of frequencies in a fixed pattern produces reception of only some of the transmissions of a trunked system. Dwelling on one channel may result in reception of only one transmission of a multiple transmission communication. Thus, a conventional scanning receiver dwelling on a channel cannot reliably indicate the source of the transmission received, e.g., police, fire, rescue, etc. Moreover, since the conventional scanning receiver does not have the ability to receive, decode, and use the digital signaling information, it cannot follow the frequency changes of a particular group of transmissions, i.e., talk group. Therefore, a listener using a conventional scanning receiver can almost never hear a complete conversation in a trunked radio system.
If a conventional scanning receiver tries to monitor a trunked communication, the scanning receiver may receive a first transmission in the communication. However, the scanning receiver is unlikely to receive subsequent transmissions in communications occurring at different frequencies. For example, a scanning receiver may receive a first transmission in the trunked communication at a first frequency. When the first transmission ends, the scanning receiver may resume frequency scanning. The second transmission in the trunked communication occurs at a second frequency. However, instead of receiving that subsequent transmission, the scanning receiver may dwell on a transmission at a third frequency, most likely a transmission from another trunked or non-trunked communication system. Even if the conventional scanning receiver begins frequency scanning after receiving a first transmission and, by accident, dwells on a second frequency where the first communication continues, the scanner may be late in tuning to the second frequency so that part of the second transmission is missed. Thus, a conventional scanning receiver monitoring a trunked transmission may receive interleaved fragments of several unrelated communications from different sources or fragments of one communication.
U.S. Pat. No. 4,905,302 to Childress et al. (hereinafter Childress), the disclosure of which is incorporated by reference, describes an example of a trunked radio communications system. In the Childress system, radios send and receive digital control signals over a dedicated control channel for designating operating frequencies, i.e., working channels, for voice communications. In the Childress system, a radio initiating a communication is referred to as a calling unit. A radio to which a communication is addressed is referred to as a called unit. In order to initiate a communication, a calling unit sends a digital signal referred to as a channel request over the control channel to a control site. The control site responds by transmitting another digital signal referred to here as a channel assignment message over the control channel to the calling unit.
A channel assignment message comprises bit codes that include a channel identification code and a talk group identification code. The channel identification code indicates an assigned working channel over which a voice transmission will occur. The talk group identification code indicates the group of users intended to receive the transmission (the called units).
Once the calling unit receives the channel assignment message with a talk group identification code that matches the talk group identification code of the calling unit, the calling unit tunes to that assigned working channel for transmission and reception. The control site also sends a confirmation message on the assigned working channel. The calling unit responds by sending a verification message to the central unit over the assigned working channel. The central unit responds to the verification message by sending a command over the assigned working channel to "unsquelch" the radios of the called units, i.e., to permit reception of the voice communication. The user of the calling unit then transmits voice information. All of these exchanges are completed before voice communication and are initiated, for example, when the system user first presses the push-to-talk button on the calling unit.
Only the called units having the transmitted talk group identification code are actuated by the "unsquelch", i.e., unmuting, command and receive the transmission. Radios within the system but outside of the called talk group remain muted or tuned to the control channel for receiving other communications. Thus, the only way for a radio within the system to receive a transmission is to have a talk group identification code matching the talk group identification code in the channel assignment message of the digital control signal.
During communications in a trunked system between a calling unit and a control site, the control site continually broadcasts, on the control channel, periodic update messages. The update messages have a similar format with regard to the channel identification and talk group codes as the channel assignment message. However, the channel assignment message is transmitted only once upon initiation of communication over the working channel. The update message is repeatedly transmitted on the control channel during voice communication on the working channel so that transceivers in a talk group communicating on the working channel that are newly turned on, are re-establishing communication after a loss of signal, and the like, can join the talk group communications. These added transceivers join the communications "in progress" based upon detection and decoding of an update message. In this disclosure, references to a message, generically, encompasses both channel assignment messages and update messages.
Communication is terminated when the push-to-talk button on the calling unit is released. When this release occurs, the calling unit transmits an unkeyed message over the assigned working channel to the control site. The control site responds to the unkeyed message by sending a drop signal on the assigned working channel. Upon receiving the drop signal, the calling unit and the called units relinquish the assigned working channel and are tuned to monitor the control channel.
The process described is repeated for subsequent transmissions. For example, if a called unit responds to the first transmission from the first calling unit, the called unit becomes a calling unit and transmits a channel request to the control site on the control channel. The control site then assigns a working channel that may be different from the assigned working channel for the previous transmission. Because of the changes in frequencies, monitoring communication in a trunked system as described by Childress with a conventional scanning receiver is impossible for the reasons previously detailed.
In the Childress system, participating transceivers utilize complex bit codes and handshaking protocols. For example, communications over the control channel are synchronized using synchronization bit patterns preceding each transmission. Accordingly, mobile transceivers must include pattern recognition circuitry for recognizing these synchronization patterns.
Numerous other bit codes are also transmitted on the control channel in the system described by Childress. For example, a calling identification code may be included in each channel request signal on the control channel for identifying the specific calling transceiver. The calling identification code may also be included in the channel assignment messages. By using the calling identification code, the working channel assignment may include a command that enables only the calling transceiver to transmit on the assigned working channel. In that system, each mobile unit must include circuitry for transmitting and recognizing its identification code.
Another bit code transmitted on the control channel is a message code indicating whether the call is message trunked or transmission trunked. A third code may be a communication code indicating whether the call is to an individual radio or a group of radios. Individual radios in the Childress system must recognize and respond to each of these codes in order to communicate in the system.
In addition to transmitting bit codes over the control channel, the Childress system also transmits sub-audible data over the assigned working channel. This sub-audible data enables a higher priority communication to interrupt a lower priority communication that is in progress on an assigned working channel. Accordingly, transceivers participating in that system must include circuitry for detecting sub-audible data.
While the sophisticated trunked communication system has many operational advantages, monitoring trunked communications has required a complex trunked system transceiver and, therefore, has been expensive and lacking in flexibility. Further, conventional trunked system transceivers are arranged only to detect certain communications, under circumscribed conditions, and are restricted to a limited number of talk groups. Accordingly, there is a need for a simple receiver for passively monitoring a trunked radio communications system so that communications can be coherently monitored even when the frequencies of transmission change.